In a conventional electric power source circuit, an power supply relay is provided in an electric current path between an electric power conversion circuit and a battery to turn on and off the electric current path. In a case that the length of the current path between the power conversion circuit and the battery is long, a capacitor is conventionally provided near the power conversion circuit so that the capacitor is pre-charged by the battery to stably supply the power conversion circuit with electric power.
If the power supply relay is used, it is desired to diagnose whether the power supply relay normally operates. The power supply relay is diagnosed with respect to its abnormality (malfunction) of fixed-closure, in which the power supply relay is persistently closed due to fixation of a movable contact. This fixed-closure can be determined by detecting a charge voltage of the capacitor in response to a trigger that is caused when an on/off switch part (ignition switch) for connecting and disconnecting the battery and a part operating the power conversion circuit is closed. At the time immediately after the ignition switch is turned on, the capacitor is supposed to have not been charged yet because the power supply relay has been turned off. If the capacitor has been charged, the power supply relay is diagnosed as being in the abnormal condition of fixed-closure.
Fusion abnormality is considered as one of the causes of the fixed-closure of the power supply relay. This abnormality arises, when a large electric current flows from the battery to the capacitor at the time of turning on the power supply relay to the closed state and melts the movable contact to a fixed contact during closure of the power supply relay. It is therefore desired to avoid the fusion abnormality. A power supply relay, which can supply a large electric current, may be used to avoid the fusion abnormality. This power supply relay however is large in size and costly.
JP 11-245829 proposes to connect a capacitor connected between a H-bridge circuit of an electric motor and a power supply relay to a battery through an on/off switch part (ignition switch) and a pre-charge resistor. When the ignition switch is turned on, the charge of the battery is supplied to charge the capacitor through the ignition switch and the resistor. As a result, even when the power supply relay is turned on subsequently, a large current is restricted from flowing from the battery to the capacitor through the power supply relay.
The fixed-closure of the power supply relay may arise for causes other than the large current, which flows to the capacitor. It is therefore also desired in the proposed power source circuit to diagnose whether the power supply relay is in the fixed-closure state. In the case of an arrangement, in which the capacitor is charged by turning on the ignition switch, the capacitor may not produce a large voltage difference, by which the fixed-closure of the power supply relay is detectable, between the capacitor voltages at the time of turning on the ignition switch. It is therefore difficult to diagnose the power supply relay with respect to the fixed-closure abnormality.
It is also possibly difficult to diagnose fixed-open abnormality, in which the power supply relay is persistently held in the fixed-open state, because the capacitor is charged by turning on the ignition switch.